What is a trimmer joist?
A trimmer joist is the joist that carries the joists interrupted by an opening in a floor. That sounds more definite than it is: the term covers two different roles, and which one is meant varies between sources and between countries.
Two roles, one name
| Role | Description | Names in use |
|---|---|---|
| Continuous joist alongside the opening | Runs bearing to bearing, made heavier, carries the cross member | Trimming joist, sometimes trimmer |
| Cross member closing the opening | Runs at right angles to the joists, picks up the short joists | Trimmer, header joist |
In British usage the trimmer is usually the cross member and the trimming joist the continuous one. Other sources, including Dutch practice, sometimes assign the names the other way round: the Dutch raveelbalk is used both for the continuous joist and, by building historians especially, for the cross member, which strictly has its own name.
The practical answer is to describe the role rather than rely on the label. Say “the continuous joist alongside the stairwell” or “the cross member the short joists land on”. That removes ambiguity on site and when ordering timber and hangers.
Why it is made heavier
A trimmer joist carries a concentrated load. An ordinary joist supports only its own strip of floor; this one picks up the load of every interrupted joist as well, delivered at a few points rather than spread evenly along its length.
- Upsize it or double it. A single joist at the standard section is almost never enough.
- Fasten doubled joists properly. Two joists side by side only act together if they are connected along their length.
- Use joist hangers. For the connection of the short joists to the cross member, hangers are the most reliable answer.
- Check the bearing. The extra load ends up on the wall or on a beam below, which has to be able to take it.
- Have the section calculated. The right size depends on span, loading and timber grade, and should be determined by an engineer.
Related terms
- Trimmed opening
- Trimming joist
- Header joist
- Binder
- Joist hanger
- Floor joist
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